I am looking for a reference that has a proof that a path groupoid is a groupoid internal to the category of diffeological spaces. I do know how to prove this fact, and a proof is not hard. My reason for asking this question is that I don't want to clutter a paper with things that are apparently well-known. However the nLab article linked to above gives no references.
The places I tried to look this fact up are: Diffeology by Patrick Iglesias-Zemmour, papers of Schreiber and Waldorf, papers of Picken with various collaborators, the nLab, google and google scholar.